1. Field of the Invention
The present invention relates to an imaging forming apparatus used as an electrophotographic color copier and a color printer and an image forming method using the image forming apparatus.
2. Related Art
Even in a color copier and a color printer, a strong tendency to color images is seen lately. The color image forming methods, which are valuable for practical use, are broadly classified by generally used popular names into the intermediate transfer method, the KNC method (a method for forming a multi-color superimposed image on an electrophotographic photoconductor and transferring it in a batch), and the tandem method.
Although these are of course popular names given from different viewpoints, for example, there is naturally a method which is the intermediate transfer method and also the tandem method. A color image forming apparatus of the intermediate transfer method and the tandem method (hereinafter, the tandem method is referred to as the method which is the intermediate transfer method and also the tandem method) is known as a one for producing full color images of high quality. This method forms toner images on the respective electrophotographic photoconductors (hereinafter, referred to as just photoconductors) corresponding to the colors of yellow, magenta, cyan, and black, forms a color superimposed image on an intermediate transfer medium, and transfers it onto a transfer material in a batch.
In the color image forming by the tandem method, there are transfer steps of two stages of primary transfer of transferring toner images from each photoconductor to the intermediate transfer medium and secondary transfer of transferring the toner images from the intermediate transfer medium to a recording paper, and there are two stages of cleaning such as cleaning of the photoconductors after the primary transfer and cleaning of the intermediate transfer medium after the secondary transfer, so that defective images due to defective transfer of toner images or defective images due to cleaning are often apt to be formed.
In the image forming apparatus using the intermediate transfer medium, to avoid enlargement of the apparatus, a method adopting a belt-shaped intermediate transfer medium capable of realizing miniaturization of the apparatus is often used. However, in cleaning of the belt-shaped intermediate transfer medium, the contact status of a cleaning member with the intermediate transfer medium is apt to be changed and defective cleaning is easily caused. To solve the defective cleaning of the intermediate transfer medium, several proposals have been made. For example, a method which uses a cleaning device (Japanese Patent Application 2002-162839) with a cleaning blade adhered to a thin metallic plate member or a cleaning device (Japanese Patent Application 2002-278319) using a cleaning blade and a metallic scraper and removes residual toner on the intermediate transfer medium has been proposed.
However, when in the particle size distribution of toner of a developer, a large amount of toner component of fine particles with a diameter of 1 μm or so is contained, the fine particle toner component is easily adhered to the surfaces of the photoconductor and intermediate transfer medium and defective transfer and cleaning are easily generated from the photoconductor or intermediate transfer medium. Particularly, when fine particle toner is adhered to an intermediate transfer medium having small surface wear, partial toner filming occurs on the intermediate transfer medium, and the toner filming part becomes unstable in the contact wear with the cleaning member, and even if the aforementioned cleaning device using a cleaning blade and a metallic scraper is used, the intermediate transfer medium cannot be cleaned sufficiently, and toner easily passes through the cleaning blade, and as a result, image defects such as uneven images are easily caused.
Further, in defective transfer of toner from the photoconductor to the intermediate transfer medium, image defects such as reduction in the image density and missed transfer are easily caused. On the other hand, in defective transfer of toner from the intermediate transfer medium to a recording paper, scattered letters due to flicked transfer and reduction in sharpness are reported.
To improve the transferability causing “missed transfer” and “scattered letters”, the arts of making the surface layer of an electrophotographic photoconductor contain fine particles, making the surface uneven, reducing the adhesive force of toner on the surface of the photoconductor, improving the transferability, and reducing the frictional force with the blade have been examined. For example, it is reported that a photosensitive layer contains fine particles of alkylsilsesquioxane resin (Japanese Patent Application 5-181291). However, fine particles of alkylsilsesquioxane resin is hygroscopic, and the wettability of the surface of the photoconductor, that is, the surface energy is increased in a high humidity environment, so that a problem arises that the transferability is apt to reduce. On the other hand, to reduce the surface energy of the photoconductor, an electrophotographic conductor containing fluorine plastic powder is reported. However, sufficient surface strength cannot be obtained by fluorine plastic powder, and a streak fault caused by a scratch of the photoconductor surface is easily generated, so that a problem arises that image diffusion is easily caused (Japanese Patent Application 63-56658).
On the other hand, to improve the transferability of the intermediate transfer medium and the cleaning performance, an art of supplying a solid lubricant to the intermediate transfer medium and reducing the surface energy of the intermediate transfer medium is published (Japanese Patent Application 6-337598, Japanese Patent Application 6-332324, Japanese Patent Application 7-271142). However, only such control for the surface of the intermediate transfer medium is not sufficient for improvement of the total transferability of the image forming method using the intermediate transfer medium having the two transfer steps and particularly for copy image forming at high temperature and high humidity or over a long period of time, it is found that further improvement is necessary.
On the other hand, taking notice of the electrophotographic process, the latent image forming method is broadly classified into analog image forming using a halogen lamp as a light source and image forming of a digital method using an LED or a laser as a light source. In recent years, as a printer of a personal computer or even in an ordinary copier, from easy image processing and easy development to a copier, the latent image forming method of the digital method is suddenly used predominantly.
In image forming of the digital method, the use method for not only copying an image but also forming an original image is increased and in electrophotographic image forming of the digital method, a tendency to high image quality is seen.
For the aforementioned request for realization of high image quality, a study of faithfully developing a latent image on an electrophotographic conductor has been made using polymerized toner controlling the shape factor and particle size distribution. However, when the polymerized toner is used for a color image forming apparatus of the tandem method, improvement of the toner transferability and improvement of the cleaning performance are not realized so much as expected at first and image defects due to defective transfer and passing of toner due to defective cleaning are easily caused.
Particularly, in the color image forming apparatus of the tandem method using the intermediate transfer medium, it is found that improvement of the total toner transferability of the aforementioned primary transfer and secondary transfer and improvement of the cleaning performance of the intermediate transfer medium are important.
A main object of the present invention, with the foregoing problems in view, is to provide an image forming apparatus and an image forming method using a useful intermediate transfer medium. Further, another object of the present invention viewed from one aspect is to provide good electrophotographic color images using an image forming apparatus using an intermediate transfer medium and to provide an image forming apparatus of an electrophotographic method and an image forming method, particularly in forming a large number of color images, capable of improving the transferability from a photoconductor to an intermediate transfer medium, the transferability from the intermediate transfer medium to a recording material, and the removability of the residual toner component on the intermediate transfer medium, thereby preventing missed transfer and scattered letters due to defective transfer or uneven images due to defective cleaning, and reproducing color images of good sharpness and a clear hue. Still another object is to provide an image forming apparatus of an electrophotographic method and an image forming method, particularly using a belt-shaped intermediate transfer medium, capable of preventing uneven images due to defective cleaning easily caused by the intermediate transfer medium and missed transfer and scattered letters due to defective transfer and reproducing color images of good sharpness and a clear hue.